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Adopting a new image processing technique astronomers have discovered some of the most distant supernovae, paving the way to reveal the very first stellar explosions.

Supernovae result from the explosions of massive stars – more than eight times the mass of the Sun – as they reach the ends of their lives. "When stars explode, they spew matter into space. Eventually, gravity collapses the matter into a new star, which could have planets such as Earth around it," says Jeff Cooke of the University of California.

Cassiopeia A is one of the most well known supernova remnants. Hot gas was created when ejected material from the dying star smashed into the surrounding gas and dust at high speed. Image: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech.

Cooke lead the study of larger stars – 50 to 100 times the mass of the Sun – that eject part of their mass into their surroundings before they die. When they finally explode, the remaining material ploughs into the previously expelled gas, lighting it up like a beacon for many years.

Typically, astronomers find supernovae by comparing pictures taken at different times of the same swath of sky and looking for changes. Objects that appear to brighten and then fade over time represent characteristic behaviour of a star that has blown apart and died.

Cooke built upon this idea and stacked blended images from the Canada-France-Hawaii Telescope (CFHT) Legacy Survey taken over the course of a year, and compared them with image compilations from other years. "If you stack all of those images into one big pile, then you can reach deeper and see fainter objects," says Cooke. "It's like in photography when you open the shutter for a long time. You'll collect more light with a longer exposure."

GRB 090423 was spotted by NASA's Swift Satellite earlier this year and is the most distant object of its kind. Image: NASA/Swift/Stefan Immler

By comparing the composite images over the four years, Cooke’s team identified four potential supernovae, which were followed up with observations by the Low Resolution Imaging Spectrograph (LRIS) on the Keck I telescope and the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope. By analysing the spectrum of light that each object emitted, the astronomers could determine the objects' composition and distance, placing them at a distant 11 billion years.

These rewrite the record books for the most distant supernovae; the previous record holder of the same type of supernovae occurred six billion years ago. Of course, the recent discovery of gamma-ray burst 090423 (read more) is the ultimate record breaker, located at 13 billion years and therefore seen when the Universe was less than 700 million years old, but only the most energetic stellar explosions can produce gamma-ray bursts in their final death cry.

Studying the deaths of these early stars is essential to understanding the evolution of the Universe and how its elements were formed and distributed to create stars, planets and even life, throughout the cosmos. With a new era of large survey telescopes soon to go online, astronomers are well on the road to observing the death of the very first stars of the Universe.

The new method and first results appear in the 9 July edition of the journal Nature.

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